Acoustic resistance device



Dec 29,1936, M. w. SCHELDORF ACOUSTIC RESISTANCE DEVICE Filed Dec. 31, 1935 2 Sheets-Sheet 1 fair Harv/21L 5M Dec. 29, 1936. M. w. SCHELDORF ACOUSTIC RES I STANCE DEVICE Filed Dec. 31, 1955 2 Sheets-Sheet 2 l'ob la'oo FREQUENCY lA/ CYCLES/ 51? GAL-COND- nI/aniar w 4 WV V 6 w 3 4 l a 35k a 9 1 1 .l 5 L 2 Z 4 Patented Dec. 29, 1936 UNITED STATES ACOUSTIC RESISTANCE DEVICE Marvel W. Scheidorf, Haddon Heights, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 31, 1935, Serial No. 56,989

20 Claims.

My invention relates to sound translating apparatus and more particularly to acoustic resistance devices for use in such apparatus.

In sound translating apparatus, particularly in acoustic cabinets employing a. direct acting or piston type dynamic loud speaker, mounted in an opening in the cabinet, considerable difficulty has been caused by reflections of sound waves etween the walls of the cabinet and the rear 1 of the speaker diaphragm, resulting in excessive peaks and dips in the response curve. More particularly, there has occurred undesired cabinet resonance or overemphasis of the lower audio frequency portion of the sound wave spectrum. In the case of small, substantially enclosed baiiles, e. in automobile speaker housings, considerable diiiiculty from undesired resonance has resuited from the fact that the natural period of the speaker diaphragm assembly and the efiect of the small casing enclosure have been about equal, thereby increasing the tendency toward distortion and howling.

Various attempts have been made to overcome this difiiculty, for example, resonance chambers have been employed within cabinets for absorbing sound at the frequencies of the objectionable disturbances. These means have proven satisfactory but have been rather expensive and critical in design because it has been necessary to tune the resonance chambers. Other attempts have been made, in the direction of damping, as by means of placing a substantial amount of sound absorbing material inside of the cabinet. This means has been found effective in preventing reflection of high frequency sound waves but has proven inefiective to damp the low frequencies and prevent cabinet resonance or boominess.

It has been proposed by certain workers in the art to employ what is known as acoustic resistance material in the cabinet space at the rear of the loud speaker diaphragm in order to overcome the above difiiculty. For example, Flanders et al. Patent #1,841,101, discloses an acoustic resistance structure of thin sheet metal containing narrow slits, the sheet covering the rear side of a speaker cabinet.

It is, accordingly, an object of my invention to provide an improved acoustic resistance structure for improving the performance of sound translating apparatus.

It is a further object of my invention to provide improved sound translating apparatus utilizing my new and improved acoustic resistance device.

The novel features that I consider characteristic of my invention are set forth with par-- ticularity in the appended claims. The inven-:

tion, itself, however, both as to its organization and its method of operation, together with addia tional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which Fig. 1 is a side view, partly in section, of a cabinet and loud speaker device, shown for the purpose of illustrating my invention,

Fig. 2 is an equivalent electrical circuit corresponding to the acoustics of Fig. 1,

Fig. 3 is a view corresponding to Fig. 1, of a cabinet and speaker, with my improved acoustic resistance structure mounted in the cabinet.

Fig. 4 is an equivalent electrical circuit corresponding to the acoustics of Fig. 3,

Fig. 5 is a side elevational view, partly in section, of a dynamic loud speaker and cabinet housing, embodying more in detail the novel features of my invention,

Fig. 6 is an inside view of the rear wall of the cabinet shown in Fig. 5 viewed in the direction of the arrows 6-6, and showing more in detail the acoustic resistance arrangement made in accordance with my invention,

Fig. 6a. is a plan view of a slightly modified form of my acoustic resistance structure,

Fig. 7 shows comparative characteristic curves of acoustic characteristic of a cabinet and loud speaker, with and without my invention,

Fig. 8 shows impedance curves of a loud speaker circuit in combination with a speaker cabinet with and without my invention,

Fig. 9 is a rear view in elevation of an open back acoustic cabinet and speaker with modified forms of my invention in combination therewith,

Figs. 10 to 14 show, in greatly enlarged form, plan views of wire gauze in difierent stages of flattening, made in accordance with my invention, and

Fig. 15 is a side view in elevation of an acoustic cabinet and speaker embodying modified forms of my invention.

Referring to the drawings for a more complete understanding of my invention, a resonator method of eliminating cabinet resonance is indicated in Fig. 1, reference being madeto Carlisle et al. Patent 711,837,755 and Wolfi Patent #1301388. A piston or cone type diaphragm loud speaker I is shown mounted in a closed cabinet 3 which acts as a baffle, preventing direct circulation of sound waves between the front and rear surfaces of the diaphragm 5 of the speaker. A partition 7 is disposed in the cabinet and is provided with a restricted communication passage 9. This partition divides the cabinet into a diaphragm compartment H and a resonator cavity i3. The opening 9 acoustically couples the cavity I3 to compartment H, and is ordinarily covered by a sheet of sound damping material, such as cloth, as indicated at 15, for increasing the resistance of the passage. The corresponding electrical equivalent diaphragm is shown in Fig. 2.

Another enclosed cabinet 3 with a loud speaker I mounted therein, are shown in Fig. 3 together with my improved acoustic resistance device I! mounted in an opening id in the rear wall 4 of the cabinet, the equivalent electrical circuit being shown in Fig. 4.

Referring to Figs. 2 and 4, S is a source of oscillatory acoustic energy; L5 is inductive reactance of the cone diaphragm system plus inductive air reaction: Rm is resistance of the air being driven by the cone plus losses in the cone; C5 is the capacitive reaction of cone diaphragm suspension and center; C11 is the capacitive reaction of the speaker compartment ll; C13 is the cap .citi e reaction of cavity (3; L9 is the inductive reaction of the opening 9; R9 is the resistance of the opening 9; L17 is the inductive reaction of the screen or wire gauze I1; R17 is the resistance of screen 57.

The first method is found to be the most effective if the resistance Rs approaches the value 5 1 C13 especially when L9 and L5 are of the same order. In this connection it is interesting to note that the resonator would not be needed if one increases Rail by resistance damping to approach a value 55 5 In both of these cases it is difiicult to get suff1- cient damping. Therefore, the results are not always satisfactory.

My improved method works proaches a value where R17 is large compared to L17 and L17 is small compared to L5. The effect of the shunt circuit in this case is to minimize the effect of C11 so that we are in effect simply adding damping to the original cone diaphragm system.

Heretofore attempts have been made to obtain acoustic resistance effects by the use of silk or other cloth, and by means of woven wire screen or gauze of fine mesh. Cloth has the disadvantage that it isnot permanent in nature and the resistance offered varies with atmospheric conditions, particularly moisture. A greater disadvantage is that it lacks rigidity; it is important that the resistance material remain stationary and that the rapidly moving air surge through the restricted openings in the material with resulting losses and damping. Wire gauze heretofor employed has been known to offer a certain best when R17 apamount of acoustic resistance, but it has not been substantially pure resistance and has not been available in the proper form to give the desired results, nor have all the determining factors been fully appreciated by others. While it is possible to provide acoustic resistance by means of a thin metallic sheet having a plurality of openings formed therein, as by a chemical etching or other special process, there is no known process for producing such an arrangement in an inexpensive and simplified manner, such as would be adaptable to quantity production of sound translating apparatus.

Satisfactory results are not obtained with ordinary wire screen materials because they do not cause enough resistance compared to the reactance equivalent in view of the thickness of the screen in terms of mesh size. In accordance with my invention I have found that resistance of a screen is increased many times and made more from the fact that resistance varies approximately inversely as the average diameter of aperture cubed, while reactance varies only as the inverse of the aperture diameter to the first power. I have found that apertures other than circular in shape are preferable for a given opening area for the production of pure acoustic resistance. The edges of the opening are desirably sharp and extensive in length, opposed edges being preferably closeiy adjacent. A desirable form of mesh opening is a narrow slit, as suggested by the Flanders patent. Between and around the opposed edges the air particles are forced to surge in volume at high velocity, dissipating energy in the resistance to the passage of the air by friction and collision.

Further, in accordance with my invention, I have provided an acoustic resistance device which is easily and inexpensively manufactured, is durable, and has substantially improved acoustic resistance properties by reason of the extreme fineness of mesh and thinness of the screen, as well as rigidity, which properties and characteristics are essential to an eiiicient and commercially successful acoustic resistance device.

Referring to Figs. 5 and G, by way of illustration of my invention, data was secured with a radio type l tL- iiAl speaker i, mounted behind an opening in the front wall of a box 3 having lateral walls i2. and inside dimensions lO.5x-lO.5:;6.5 inches. The speaker i moving coil electrodynamic type inch diameter cone 5 flexibly su ten inch diameter cone 5 A cone bracket 8 connects the m G and electromagnet field structure 8. The ace c resistance filter consisted of two pieces of soi! in the rear wall 4 of the box, ach scre ling 5x125 inches nded from a eifective The e11 SOlQvziGd to a copper frame naving a -dow 5x125 inches,

rinining the effec- .005 inch in width to .002 inch. The frame L! was used as a convenient mounting for securing the screens I! to the rear wall 4 of the cabinet over the openings ID. The baffie was sealed against air leakage, except through the resistance screen, thereby improving the effectiveness of the screen.

The above arrangement gave desirable results that will be seen by referring to the curves in Fig. '7 plotted between cycles per second (abscissa) and sound pressure output (ordinate) in arbitrary units. Curve A represents results with the acoustic resistance filter, and curve B without it. Corresponding curves A and B in Fig. 8 were taken between impedance in ohms (ordinate) of the voice coil circuit of the speaker and the frequency in cycles per second (abscissa). The peak in curve B shows the reflected impedance caused by the undesired peak in curve B, Fig. 7. This reflected impedance represents distortion in the amplifier output circuit and is particularly bad in pentode or high impedance output tubes. It will be observed that curve A is slightly higher throughout the normal response range than B. This is caused by suppression of the resonance peak, and in practice is more pronounced than is shown by the curves; at excessive resonance (curve B) the voice coil movement exceeds the normal limit of the air gap, and while, in this condition, other superimposed frequency impulses are impressed at instants when the coil is not in the maximum magnetic field, thereby lowering the response.

An advantage of my improved acoustic device is the simplicity with which a result can be secured. Resistance units are not critical, tuned circuits are. After data is secured on screen resistances, calculations are simple for a given resonant condition. Secondly, I secure the same result in less space, or better results in the same space; resonators require the extra volume of 20-30%. The size feature affects the weight also and both are important with portable loud speakers.

Referring to Fig. 9, I have illustrated an application of my acoustic resistance devices in a loud speaker cabinet that is open at the rear, comprising only the front wall 2, and lateral walls l2. In such case the resonant tendency is materially reduced. In this arrangement, a plurality of small leakage openings have been provided in the front wall 2 of the cabinet or baflie, preferably closely disposed around the speaker, each of the openings being covered with my acoustic resistance device II, that may be mounted on a circular frame 23 corresponding to frame I!) of Fig. 5. The rear may be completely closed if desired, relying on the front openings to prevent cabinet resonance. The low frequency waves in passing through the screens are shifted in phase and reinforce the front waves to a certain extent. This effect is accentuated by making the screen frames 23 in the form of cylinders as in Thuras Patent #1,869,1'73.

By way of illustration of another application of my invention, I have shown also in Fig. 9 what may be termed a built-in speaker, consisting of a sheet metal speaker cone housing or partition 2| having sound openings 25 therein at the rear of the cone diaphragm and forming a diaphragm compartment within the larger cabinet enclosure. The openings are covered by my acoustic resistance material II, as in Weinberger et al., Reissue 18,751, for the purpose of filtering the disturbance at the source and keeping it out of the cabinet. It also acts as a screen to protect the rear of the diaphragm from sound impulses reflected from the cabinet walls or from some other source, as from another loud speaker. With this arrangement, because of the relatively close confinement of air back of the diaphragm by the housing partition 2|, it is essential that the total resistance offered by resistance devices I! to the movement of air through the openings 25 be not so great as to undesirably retard the normal movement of the diaphragm at the low frequencies where the movement is relatively great. For best results, the total area of the openings 25, and hence the total area of mesh screen, as well as the individual apertures or pores in the wire mesh should be properly proportioned.

Inasmuch as the cone housing 2| is located in a high pressure low velocity sound area, it is desirable for proper results that the acoustic screen be high in resistance, in other words, the screen resistance should match the sound pressure at the position located. A low resistance screen is used in a low pressure area. In order to prevent undue restriction on the normal diaphragm movement in the foregoing example, it is desirable that the total area of the acoustic screen, or the housing opening areas 25 be large and properly distributed, While for proper damping, the mesh apertures of the screen should be small. If desired, the openings 25 in the partition or dishpan type housing 2| may be enlarged, or the dishpan may be dispensed with, and nothing but screen resistance material provided in about the same position, thereby obtaining a greater total distributed screen area directly behind the diaphragm. The foregoing arrangement may be used singly with a speaker, or in combination, with acoustic resistance damping also in the cabinet walls as shown in Fig. 9, the effectiveness of successive damping being desirable in some cases.

While a very fine mesh screen of high specific resistance per unit area can be concentrated at a relatively small opening area where the enclosure back of the diaphragm is not too confining, as in Fig. 5, it is desirable to provide a more distributed acoustic resistance when used close to the diaphragm as in Fig. 9. It appears desirable for the purpose of rigidity and to prevent vibration of the screen to break up a given required area of acoustic resistance into smaller areas with surrounding support, as in Fig. 9, aside from the above question of distributed resistance. While in Fig. 5, I have shown separate screens, the area was not so great but that a single larger screen area could have been employed to advantage, as in Fig. 6a. In Fig. 60., I have shown a pair of ribs 20 having inwardly turned flanges 22 for adding rigidity to the screen, if desirable, although ordinarily this expedient is not necessary with my improved resistance material. As resonance the travel of the diaphragm tends to become excessive and to pump a considerable volume of air. It follows that a substantial amount of rigidity is essential in acoustic resistance material in order to be effective. In order to further strengthen the screens against vibration by high pressure sound waves, the screens may be stretched on their frames, in Figs. 6, 6a, and 9 by any suitable means. A strong circular frame is best suited for this purpose. The screen may be heated and, in this condition, brazed to the frame. On cooling, the screen shrinks and hence stretches.

Referring to Figs. 10 to 14, I have shown, by way of example of structure made in accordance with my invention, a 100 mesh wire screen in different degrees of thickness as a result of rolling,

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whereby different acoustic resistances are obtained. Fig. 10 shows in substantially enlarged form a 100 mesh screen prior to rolling. Each of the rectangular space 29, bounded by wire mesh strands 33 and El, measured about 4.5 by 4.5 mils, the dimensions of which may be termed the length and width, particularly as the apertures become elongated. The strands 38 running lengthwise of the rectangular apertures in the direction of the arrow 35 may be termed the warp, while those 3i running crosswise of the apertures may be termed the w It will be convenient in some cases to use the term average diameter, corre sponding to the diameter of a circle of the same area as that of the rectangle, a term that is sufficiently accurate when the length width are not greatly different. The wire strands 30 and Si measured 5.5 mils diameter in section and the maximum thickness of the mesh screen measured 9.5 mils. The acoustic resistance may be represented by K, a figure that is proper icnal to the mechanical ohm defined by Flanders etal. in the above patent.

In the following table I have listed the results of l for the different degrees -olling.

Rollinc- F1 g was Opening Thick K D ncs A Mrs Itis noted that after a li-.-ited amount of rolling the wires begin to flatten, for example at 33 in Fig. 11, at the points of crossover until the strands become thin strips, as in Fig. 14. The rolling was in the direction indicated by the arrow 35, resulting in the rectangular spaces being elongated in the same direction, thereby forming elongated aperatures or slits, a desirable form for good results, as above disclosed. It will be observed that the factor K, corresponding to the acoustic resistance, increased rapidly with the decrease of mesh aperture dimensions, particularly the width, and thickness of screen. In the above examples the thickness of the screen after rolling is comparable to the distance between the closer of the opposed edges of the rectangular apertures or slits. It will be noted in the case of Rolling #2 the resistance increased while the length of the apertures increased, the determining factor being the decreased thickness of screen.

While I have disclosed the flattening of the wire by means of a rolling process, it is quite obvious that other flattening processes may be employed. For example, the screen may be flattened between plates of a press. While the above example was taken on the basis of ordinary commercial 100 mesh creen wire, it is preferable that special wire be obtained for this process, it being desirable in some cases that finer wire and larger mesh be employed to permit increased acoustic resistance. Some of the rolled mesh wire employed is so fine in mesh after the flattening operation that the surface has the general appearance of continuous sheet material.

Instead of flattening a screen of circular section wire, the may be formed by originally making the screen as weavii with thin metallic strips ribbons, in closely spaced edge-to-edge relation, thereby obviating the rolling process, unless it is desired to roll the ribbon structure after assembly. The lattice network of Fig. 14

so closely suggests such an arrangement that additional illustration appears unnecessary. It is desirable that the apertures be long and narrow and that the ribbon be thin. The slits may be much longer by this process than shown in Fig. 14, from rolling. The width of the slit becomes the controlling factor as far as the length is concerned, for a given thickness of screen. It is not essential that the woof be like the warp, the more important of the two in enlarged apertures; it serves mainly as a spacing and supporting ele ment for the warp and may be any desirable shape such as circular in section.

The above arrangements are essentially a lat tice network, and while weaving is the most desirable method for securing the cross wires or ribbons together, other methods within scope of my invention may occur to those skilled in the art.

In general, my improved acoustic resistance structure may be employed to advantage wherever cloth and the like have been heretofore used for similar purposes. For example, it may be used across the opening 9 in the resonator cavity of the cabinet shown in Fig. l in place of cloth heretofore used. It may be employed in ribbon micro"- phones of the velocity type, e. g. shown and claimed by Olson Patent #1,885,001, in place of a bolt of silk cloth heretofore used. Among other advantages a metal screen may be easily cleaned of particles that may tend to clog the pores, as by an air blast.

In Fig. 15 I have shown a further modification of my invention. A speaker I, which may desirably be provided with a diaphragm 351 haw ing a single logarithmic spiral groove 4|, as shown and claimed in my copending application, Ser. No. 34,065, filed July 31, 1935, is mounted in a closed cabinet 3. Sound absorbing material 43, such as felt 43 is disposed around inside the walls 2, 4, and 12 to prevent reflection of higher frequency sound waves. One or more screens 45, 4! are disposed in series relation across an opening in one of the walls for damping certain low frequency waves for which the damping material 43 is not effective. The screens 45 and 4! may be of difi'erent mesh, respectively, e. g. 45 may be large and 41 small mesh, to produce certain desired filtering effects. The combination of the different damping means for the respective portions of the acoustic range is desirable in certain high fidelity work.

In case large screens are employed, it is desirable to insure against vibration or rattling caused by high pressure sound waves. For this purpose the screen may be formed up into certain rigid configurations such as cones, or it may be tightly stretched to an extent that its natural period of vibration lies above the audible or useful range of frequencies. By way of illustration, I have shown the screens 45 and 4? in secured to the ends of a metal cylinder 45, as by soldering or brazing. The screens may stretched to any desired degree, as by means of a tensioning member, such as a bolt 48, connected between centers of the screens. In the case of one screen, the tensioning member may be attached from the screen to any desirable stationary support in the cabinet. The cylinder may be of any desired size and may be so proportioned as to produce certain desired acoustic effects in combination with the screens. The some means for stretching is useful in case it is desirable to dispose a large screen across substantially the entire area of the rear of the cabinet.

The invention has been described with reference to certain specific structures, but it will be understood that it is capable of many other modifications within the spirit of my invention and within the scope of the following claims.

I claim as my invention:

1. In combination with a direct acting acoustic diaphragm, means for actuating said diaphragm for the radiation of sound waves, means providing an enclosed baffle around said diaphragm to prevent interference between sound waves generated by the front and rear surfaces of said diaphragm, and relatively rigid means disposed in said baflle for Offering substantially pure acoustic resistance to the passage of sound waves and characterized by a sheet of woven metal strands having a multiplicity of small apertures, the thickness of said sheet being of the same order of magnitude as the minimum distance between opposed edges of said apertures.

2. An acoustic resistance device comprising a sheet of flattened wire gauze wherein the thickness of the sheet is comparable to the average diameter of the apertures in the gauze.

3. An acoustic resistance device comprising a sheet of woven thin strips having a multiplicity of elongated apertures, characterized in that the average thickness of said strips is smaller than the width of said apertures.

4. The method of making an acoustic resistance which comprises flattening a sheet of wire gauze to decrease the thickness of said sheet and the dimensions of the mesh apertures therein, discontinuing the flattening operation when the dimensions correspond with a desired acoustic resistance value, and forming a given area of acoustic resistance structure.

5. In combination with a direct acting piston type acoustic diaphragm, means for actuating said diaphragm for the radiation of sound waves, means providing a substantially enclosed baffle around said diaphragm to prevent interference between sound waves generated by the front and rear surfaces of said diaphragm, and means disposed in said baffle for offering substantially pure acoustic resistance to the passage of low frequency sound waves, said means being characterized by a lattice network of metallic strips having a multiplicity of narrow apertures, said strips being so thin that the ratio of acoustic resistance to equivalent reactance is greater than unity.

6. The invention as set forth in claim 5 characterized in that means are provided for stiffening said network to prevent vibration by high pressure sound waves.

'7. The invention as set forth in claim 5 characterized in that means are provided for stretching said sheet whereby its natural period of vibration lies above the useful audio frequency range.

8. In combination with a direct acting acoustic diaphragm, means for actuatin said diaphragm for the radiation of sound waves, means providing a baffle including front and lateral walls disposed around said aotuating means and dia- 7 phragm, and acoustic resistance material comprising a lattice network of thin metal strips disposed in said front baiil-e wall around said diaphragm and in the path of sound waves for damping resonance,

9. An acoustic resistance device comprising a sheet of lattice network having a multiplicity of restricted apertures and a metallic frame for mounting said sheet, network being characterized by a thickness that is smaller in dimension than the average diameter of said apertures.

10. Inco mbination with a direct acting acoustic diaphragm, means for actuating said diaphragm for the radiation of sound waves, means providing an enclosed sealed bafiie around said diaphragm to prevent interference by the sound waves generated by the rear surface of said diaphragm, and means disposed in an opening in said baffle for offering substantially pure acoustic resistance to the passage of sound waves of relatively low frequency, and means disposed within said bafile for preventing objectionable reflection of relatively high frequency sound Waves.

11. The invention as set forth in claim 10 characterized in that said acoustic resistance means comprises a, metallic sheet of lattice network having a ratio of acoustic resistance to equivalent reactance that is greater than unity.

12. An acoustic resistance device comprising a sheet of wire gauze, and a frame for mounting said sheet for high resistance passage of sound waves or air through said frame and sheet, said sheet being tensioned on said frame to prevent vibration by high pressure sound waves.

13. An acoustic resistance device comprising a frame, a plurality of openings in said frame for the passage of sound waves, acoustic resistance material comprising metallic lattice network, characterized by a resistance, to reactance ratio greater than unity, disposed across said openings, said frame around said openings serving to strengthen said acoustic resistance material against vibration by high pressure sound waves, and the total area of said openings being sufficiently large for a given mesh opening in said screen material, to offer the desired acoustic resistance.

14. In combination with a direct acting acoustic diaphragm, means for actuating said diaphragm for the radiation of sound Waves, and acoustic resistance material comprising a sheet of flattened wire gauze closely disposed around the rear of said diaphragm for damping sound at frequencies corresponding to an undesired resonant condition.

15. In combination with a direct acting acoustic diaphragm, means for actuating said diaphragm for the radiation of sound Waves, means for mounting said diaphragm and actuating means in a cabinet enclosure, and acoustic resistance means having a resistance to reactance ratio greater than unity comprising a sheet of woven metallic strands closely disposed around the rear of said diaphragm for loading said diaphragm at certain frequencies corresponding to an undesired resonant condition.

16. The invention as set forth in claim characterized by additional acoustic resistance material of similar material disposed in an opening in a wall of said cabinet for assurance against cabinet resonance.

17. In a sound translating system, a vibratile diaphragm assembly, a diaphragm compartment enclosing the rear of said diaphragm and having a resonant tendency of about the natural period of said diaphragm assembly, the walls of said compartment comprising acoustic resistance material of rolled wire gauze having a resistance to reactance ratio greater than unity.

18. In combination with an acoustic baflle cabinet and a built-in loud speaker having a direct acting piston type diaphragm, of a partition forming a diaphragm compartment enclosing the rear of said diaphragm with respect to the cabinet, said partition, comprising a metallic lattice network having, by reason of thinness of mesh and whereby objectionable acoustic resonance is eliminated.

20. An acoustic resistance device comprising a sheet built up by a plurality of individual thin fiat strips, separate cross members attached to and supporting said strips in closely spaced edgeto-edge relation, and forming with said strips a plurality of restricted sound passages characterized by a ratio of acoustic resistance to reactance that is greater than unity.

MARVEL W. SCHELDORF. 

